In any semiconductor device, the electrical parameters of capacitance and inductance are always present and cause signal delays in addition to signal distortions. These electrical parameters are considered parasitic parameters. All signal lines have a certain capacitance; they store an electric charge when a voltage is applied. To change the voltage, the amount of charge must be changed by the flow of electric current. This flow causes the buildup of a magnetic field that induces voltage into the current-carrying line (self-inductance), resisting the current flow, and into the other lines in the proximity of line being switched (mutual-inductance) causing a current flow there. The time during which the signal transits from its old value to the new value is limited by the current flow. Thus, the transition time increases with increasing capacitance. Shorter transition times improve performance.
The design of faster devices is concerned with the reduction of the device input capacitance and with the reduction of the driving impedance. The packaging of these devices can control the signal line capacitance to the reference plane, line self-inductance, and interline capacitances and inductances. Consequently, it can control the signal distortions and the appearance of unwanted interferences.
One internal cause of degradation as the signal propagates through the package are reflections. Signal reflections can increase transition time in a circuit. Reflections may also split the signal into two or more pulses with the potential of causing erroneous switching in subsequent circuits. Therefore, it is desirable to control signal reflections.
As semiconductor device technology advances wherein devices are operating at higher speeds and the number of inputs/outputs (I/Os) simultaneously switching increases, the need to control inductance in a semiconductor device becomes more critical to control simultaneous switching noise. Switching noise is an induced voltage on the power distribution system at the circuit terminals caused by the rapidly changing current caused by the simultaneous switching of many drivers.
Switching noise is directly proportional to slew rate and is caused by the inherent inductance of the power and ground lines. The higher the inductance, the higher the change in current. It is desirable to have low power and ground inductances to reduce the slew rate magnitude, thus reducing simultaneous switching noise.
Simultaneous switching noise is a problem in any package that has a large power and/or ground inductance. In packaging design considerations, switching signal I/Os should be sandwiched between two planes, for example, two ground planes or a power and ground plane, thus providing low inductance paths for switching currents during both low to high and high to low logic transitions. The effective inductance of a switching circuit is a function of self and mutual inductance. Inductance of package interconnects affects both noise and speed characteristics of a packaged semiconductor device.
A stripline sandwiches a conductor between two planes, either power or ground. The conductor is further separated from the two planes by a dielectric, so that the resulting structure is that of a conductor sandwiched by a dielectric further sandwiched by two planes. In a stripline configuration, the inductance of the conductor can be reduced to approximately 30% of its original inductance.